JPH0767066B2 - Voltage controlled oscillator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oscillator circuit, and more particularly, to a voltage controlled oscillator that oscillates by charging and discharging a capacitor.

[Prior Art] Conventionally, as a voltage controlled oscillator (VCO) capable of controlling an oscillation frequency by an input voltage, there is one as shown in FIG. In the figure, reference numeral 20 is a VCO, and a P-type MOSFET 4 that performs a switching operation for charging the other end of a capacitor whose one end is grounded, an N-type MOSFET 5 that performs a switching operation for discharging, and a Schmitt trigger inverter. 7 inputs are connected. The output of the Schmitt trigger inverter 7 is connected to the inverter 8. The inverter 8 constitutes the output of this oscillation circuit, and a part of this output is fed back to the input of the transistor 4 for charging and the transistor 5 for discharging. Has been done. The transistors 4 and 5 are connected to the power supply line Vcc and the ground line via the current mirror constant current sources 2 and 3, respectively. The magnitude of the current in these current mirror constant current sources depends on the voltage-current conversion circuit (V / I).
It is controlled by an input voltage of 1.

Here, when the output of the Schmitt trigger inverter 7 changes from the low level (hereinafter “L”) to the high level (hereinafter “H”), the level of the terminal of the capacitor 6 that is not grounded is higher than the input threshold level Lth. When the potential V becomes low, this output becomes "H". At this time, the output of the inverter 8 becomes "L", so the transistor 4 for charging is charged.
Is on and the transistor 5 for discharging is in an off state, and current is supplied from the current mirror constant current source 2 to the capacitor 6 through the transistor 4 for charging. Therefore, the potential V increases with the passage of time as shown in FIG. When this becomes higher than the threshold level Hth when the output of the Schmitt trigger inverter 7 changes from "H" to "L", the output becomes "L" and the output of the inverter 8 changes to "H". . As a result, the transistor 4 for charging becomes "OFF" and the transistor 5 for discharging becomes "ON".
The electric charge of the capacitor 6 is discharged through the transistor 5 and the current mirror constant current source 3 for discharging. The potential V drops with time, but when V <Lth, the output of the Schmitt trigger inverter 7 changes to "H", and the potential V starts to rise again. Thereafter, similarly, the capacitor 6 is repeatedly charged and discharged, and this circuit operates as an oscillator. Since the magnitudes of the currents of the current mirror constant current sources 2 and 3 that determine the time required to charge or discharge the capacitor 6 change depending on the input voltage of the voltage-current conversion circuit 1, the oscillation frequency can be controlled by this input voltage. it can.

Similarly, when the potential V becomes higher than Hth, the fifth
It oscillates as shown in the timing chart of FIG.

[Problems to be Solved] However, in such a conventional oscillation circuit, the power is turned “ON”.
In the initial state, the potential V is the threshold level Hth.
And the upper and lower thresholds Hth, L
It may not change to the th side. At this time, the inverted output levels of the Schmitt trigger inverter 7 and the inverter 8 do not sufficiently "ON" or "OFF" the transistors 4 and 5, and oscillation does not occur. At this time, both the transistor 4 for charging and the transistor 5 for discharging are in an intermediate transition state between “ON” and “OFF”, and the current flowing through the current mirror constant current source 2 and the transistor 4 for charging. Flows out through the transistor 5 and the current mirror constant current source 3 for discharging, so that the potential V hardly changes from the intermediate state. as a result,
Oscillation does not occur. Further, at this time, the current corresponding to the input voltage of the voltage-current conversion circuit 1 passes from the current mirror constant current source 2, the charging transistor 4, the discharging transistor 5, and the current mirror constant current source 3 to the ground from the power source. In addition to flowing toward, the through current of the inverter 8 is increased, and power consumption is increased.

The present invention solves the above-mentioned problems of the prior art, and an object of the present invention is to provide a VCO that can reliably start oscillation and has a rapid rise in oscillation.

[Means for Solving the Problems] A feature of the voltage controlled oscillator of the present invention for achieving such an object is that a Schmitt trigger inverter is connected to the other end of a capacitor whose one end is connected to a reference potential line, and a power supply is provided. Side, the first switch circuit for charging the capacitor to the capacitor, and the second switch circuit for discharging the capacitor charge to the reference power supply line are connected to the other end of the capacitor, the output of the Schmitt trigger inverter. In an oscillator circuit that feeds back to the first switch circuit and the second switch circuit via an inverter and changes from one state of "ON" or "OFF" of these switch circuits to the other state, A third capacitor provided between the other end of the capacitor and the reference potential line or between the other end of the capacitor and the power supply line. And switch circuit, the input threshold Lth of the potential of the other end of the capacitor after the power is turned Schmitt trigger inverter
An initial reset circuit for turning on the third switch circuit for a period equal to or less than or equal to or higher than Hth is provided.

[Operation] As described above, the initial value of the potential V of the capacitor 6 is set to Lth or less or Hth or more during the reset period by the initial reset circuit, and the transistor 4 for charging and the discharge are discharged. Transistor 5 for
The initial state of is set to “ON” and “OFF”, or “OFF” and “ON”, respectively.

As a result, even if the power supply voltage after the power is turned “ON” is in an unstable voltage state, it is possible to reliably start the oscillation, and the reliability of the oscillation circuit can be improved.

Furthermore, since the current flows from the power supply to the ground when the transistor 4 for charging and the transistor 5 for discharging are in the transition state, power is not wasted wastefully. The power can also be reduced.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is an embodiment of an oscillator circuit to which the present invention is applied. Here, the same components as those in FIG. 4 are designated by the same reference numerals.

In FIG. 1, the difference from the prior art is that an initialization switch element 9 composed of an N-type MOSFET is connected between the grounded terminal of the capacitor 6 and the ground. This means that an initial reset circuit 10 for turning on at the start of oscillation is added.

To start the oscillation, set the output of the initial reset circuit 10 to "H", add it to the gate of the N-type MOSFET 9, and set it to "O".
Then, the electric charge of the capacitor 6 is discharged to the ground through the initialization switch element 9. The electric potential V of the terminal of the capacitor 6 which is not grounded is equal to or lower than the threshold level Lth of the Schmitt trigger inverter 7 by the discharge. The output of the initial reset circuit 10 is kept at "H" until it goes down to "L", and then this output is set at "L" to turn off the initialization switch element 9. "Schmitt trigger inverter 7"
And the inverter 8 inverts and amplifies the terminal potential 6 of the capacitor 6 at this time with a sufficient gain, and outputs "H",
Generates "L" output. Therefore, the transistor 4 for charging is "ON" and the transistor 5 for discharging is "OFF".
Then, the charging of the capacitor 6 starts. FIG. 3A shows the potential V at this time and the initial reset circuit.
The timing chart of the output of 10 is shown.
Exceeds the threshold level Hth, the Schmitt trigger inverter 7 inverts, its output becomes "L", and the output of the inverter 8 changes to "H". Oscillation starts from this. After that, the oscillation operation is performed by repeating the charging and discharging of the capacitor 6 as in the conventional oscillation circuit.

FIG. 2 shows another embodiment of the oscillator circuit to which the present invention is applied. The difference from the embodiment of FIG. 1 is that the initialization switch element 9 is connected between the terminal of the capacitor 6 on the side not grounded and the power supply. As shown in the timing chart of FIG. 3B, at the start of oscillation, the output of the initial reset circuit 10 is set to "L", the initialization switch element 9 is turned "ON", and the capacitor 6 is charged. Potential V
When H becomes Hth or more, the initial reset terminal 10 becomes “H”, the initialization switch element 9 becomes “OFF”, the discharge of the capacitor 6 is started from here, and the potential V of the capacitor 6 becomes lower than Lth. Then, the Schmitt trigger inverter 7 inverts, and oscillation starts.

[Effects of the Invention] As can be understood from the above description, according to the present invention, the initial reset circuit keeps the voltage "V" of the capacitor 6 from the "ON" time to the "reset period" regardless of the value of the voltage V of the capacitor 6 at the start of oscillation. Since the potential V is initialized to Lth or lower or Hth or higher by the switch circuit that is turned "ON", oscillation can be reliably started even when the power supply voltage after power is turned "ON" is in an unstable voltage state. Therefore, the reliability of the oscillator circuit is improved. Further, there is no useless current consumption from the power supply to the ground when the oscillation does not start, which is a problem in the conventional circuit, so that there is an effect of reducing the power consumption.

[Brief description of drawings]

1 and 2 are circuit diagrams showing an embodiment of the present invention, and FIG.
FIG. 4 is a timing chart of an embodiment of the present invention, FIG. 4 is a circuit diagram showing a conventional VCO, and FIG. 5 is its timing chart. 1 ... Voltage-current conversion circuit, 2, 3 ... Current mirror constant current source, 4 ... P-type MOSFET, 5 ... N-type MOSFET, 6 ... Capacitor, 7 ... Schmidt trigger inverter, 8 ...
Inverter, 9 ... switch element for initialization, 10 ... initial reset circuit, 20 ... VCO.

Claims (1)

[Claims] 1. A Schmitt trigger inverter is connected to the other end of a capacitor, one end of which is connected to a reference potential line,
A first switch circuit for charging a charge from the power supply side to the capacitor, and a second switch circuit for discharging the charge of the capacitor to the reference power supply line are connected to the other end of the capacitor, The output of the Schmitt trigger inverter is fed back to the first switch circuit and the second switch circuit via the inverter, and the switch circuit is switched from either "ON" or "OFF" state to the other state. In the oscillating circuit for changing to, the third switch circuit provided between the other end and the reference potential line or between the other end and the power supply line, and the other end of the capacitor after the power is turned on. The electric potential is from the Schmitt trigger inverter output low level to H
An initial reset circuit that keeps the third switch circuit "ON" for a period below the input threshold level when changing to the igh level or above the input threshold level when changing from the High level to the Low level A voltage-controlled oscillator characterized by being provided.